1. Field of the Invention
The present invention relates to an encapsulation of an electronic device. More particularly, it relates to an encapsulation by a molding process with a compound comprising polyphenylene sulfide.
2. Description of the Prior Arts
It is known to encapsulate electronic devices with synthetic resins in various fields for the purposes of electric insulation, mechanical protection and prevention of characteristic deterioration in the atmosphere of IC, transistors, diodes, coils, capacitors, resistors, varistors and connectors. Thermosetting resins such as epoxy resin and silicone resin have been used as the synthetic resin. The encapsulation has been carried out by a casting process, a potting process, a dipping process and a molding process. The molding process such as transfer molding or injection molding is the most advantageous because it has high efficiency in the operation, easiness of the handling of the synthetic resins and high accuracy. In general, such molding process requires high molding pressure and was difficult to be applied to encapsulation of fine electronic devices.
In order to overcome such difficulties, a synthetic resin having high melt flow property and low melt viscosity is required. Thermosetting resin compounds such as epoxy or silicone compounds having low melt viscosity have been developed and used for encapsulation in the molding process.
These thermosetting compounds still have disadvantages that a toxic crosslinking agent is used; a complicated formulation is required comprised of many kinds of additives; a handling of a liquid resin is troublesome; a storage stability of a compound is not satisfactory; a long molding time is required to cure resin chemically in a mold; a flash is not easily removed; and cleaning of mold is not easy. Therefore, an encapsulation with a thermoplastic resin compound has been desired.
The fact that polyphenylene sulfide is a suitable thermoplastic resin for encapsulation and various disadvantages caused by the conventional thermosetting compounds can be overcome by using thermoplastic compounds comprising polyphenylene sulfide and a filler have been disclosed in Japanese Unexamined Patent Publication Nos. 149348/1977; 22363/1978 and 66565/1978 (Asahi Glass).
The inventors have further studied industrial applications of the encapsulation with these compounds and have found that a handling of the compounds is easy and molding properties are excellent in the encapsulation process and the molded products have excellent mechanical properties and electric properties, but that, electric resistance of the molded product in humid environment is not satisfactory. That is, if the electronic part is encapsulated with a polyphenylene sulfide compound of the prior art and exposed in a high humid environment, it has been found that moisture penetrates along an interface between a lead and encapsulation material to cause electric leakage, corrosion and in extreme cases, a disconnection of the circuit.